In wireless, portable telecommunication equipment, the most bulky and expensive components include a wideband monolithic voltage controlled oscillator (VCO) and a wideband tunable monolithic band pass filter (BPF). These components require high-Q passive elements, wherein Q refers to the quality factor, because the frequency characteristics of the VCO and BPF are stabilized and loss is reduced with higher Q. However, the conventional techniques of producing a spiral inductor on a semiconductor and a varactor using a field effect transistor (FET) result in components which exhibit low-Q properties due to the metallic loss and the limited doping of the substrate. Thus, considerable interest has been focused on an active inductor having both high-Q properties and tunable inductance.
The operation of the active inductor is similar in principle to that of the active gyrator, which can be realized by connecting an inverting amplifier with a non-inverting amplifier in parallel and back-to-back. The gyrator converts a parallel-connected capacitor and resistor (C//R) into a serial-connected inductor and resistor (L-R). In this example, the Q of the active inductor is degraded by the series resistance.
Conventional active inductors are disclosed in articles entitled "Lossless broadband monolithic microwave active inductors" by S. Hara et al., IEEE Trans. on MTT, Vol. 37, No. 12, p. 1979.about.1984, December 1989, and "Monolithic narrow-band filter using ultrahigh Q tunable active inductors" by S. Lucyszyn et al., IEEE Trans on MTT, Vol. 42, No. 12, p. 2617.about.2622, December 1994.
The conventional active inductor disclosed by Hara et al. is realized in two configurations: a first employing a common gate (CG) as a non-inverting amplifier and a common source (CS) cascode as an inverting amplifier, and a second employing a CG cascode as a non-inverting amplifier and a CS cascode as an inverting amplifier. The first configuration requires a large amount of current in order to obtain a predetermined inductance because the gain of the common gate is low, and has a small Q. The second configuration requires a large number of components, for example five transistors and one load resistor, and increases power consumption because of the large drop in voltage.
The conventional active inductor disclosed by Lucyszyn et al. also uses a CG having a resistor R.sub.g as a non-inverting amplifier and a CS cascode as an inverting amplifier. However, this configuration results in an increase in power consumption. Furthermore, a plurality of voltage-controlling ports are required because the voltages between the drain and source of the respective transistors should be the same, and the drain currents of the respective transistors should also be the same so as to offset a parasitic component.
Parasitic resistance, coupling and substrate loss further complicate realizing a frequency-variable circuit on a monolithic Si substrate.